BRCA2, the second gene linked to familial breast cancer, was identified in 1995. Since that time several investigators have been working to discover its function in breast cancer, as well as in normal cells. It has been shown to have likely tumor suppressor function and is regulated in the cell cycle. However, there is still much that is unknown about this potentially important gene. A study to examine the protein-protein interactions of BRCA2 in vivo and in vitro is proposed. This will be accomplished by 1) two-hybrid yeast assays to identify candidate proteins that interact with BRCA2, 2) reciprocal two-hybrid assays to exclude self activation, 3) cloning candidate proteins with comparison to known isolates, 4) human di-hybrid assays to determine mammalian activity, and 5) co-immunoprecipitation assays/western blotting to confirm mammalian interaction. The identification of interacting proteins will help understand how BRCA2 functions and provide possible insight into alternative pathways of both tumor suppression and cancer biology. These studies could lead to a better understanding of, and possibly treatment options for, an important disease process.